Analyte concentration determination in physiological samples is of ever increasing importance to today's society. Such assays find use in a variety of application settings, including clinical laboratory testing, home testing, etc., where the results of such testing play a prominent role in the diagnosis and management of a variety of disease conditions. Analytes of interest include glucose for diabetes management, cholesterol for monitoring cardiovascular conditions, and the like. In response to this growing importance of analyte concentration determination, a variety of analyte concentration determination protocols and devices for both clinical and home testing have been developed. For example, various calorimetric or photometric test strips are known that contain one or more testing reagents associated with a testing or reaction area, where the reagent(s) turns a different shade of color depending on the concentration of a particular analyte, such as glucose in a blood sample that has been applied to the reaction area of the test strip. The blood glucose concentration is measured by either comparing the color to a color chart or by inserting the strip into a meter such as a reflectance photometer or the like, which automatically determines the concentration from the change in color caused by the reaction between the testing reagent(s) and the analyte. Typically, a test strip, and more particularly a colorimetric or photometric test strip includes (1) a substrate including one or more reaction or testing reagents, i.e., a reaction area, (2) a support layer which provides structural support to the strip and oftentimes includes an aperture therethrough for viewing the substrate, and (3) a material that assists in the transfer of sample to the reaction area, i.e., a transferring or filtering material or structure.
However, to determine the concentration of an analyte in a physiological sample, a physiological sample must first be obtained and brought into contact with a reaction area of the test strip so that the physiological sample, and more particularly the analyte of interest or derivative thereof, may react with the testing reagent(s) associated with the reaction area. In order to obtain an accurate measurement of the particular analyte(s) of interest, a minimum sample volume must be applied to the reaction area. It can be appreciated that inaccurate measurements can result in serious and even life-threatening consequences for those whose lives depend on frequent monitoring of an analyte in their body, for example glucose monitoring for diabetics.
FIGS. 5A and 5B show views of an exemplary, conventional test strip. FIG. 5A shows an exploded view of a conventional test strip configuration and FIG. 5B shows the configured test strip of FIG. 5A. Test strip 300 includes, as described above, a support layer 306 having an aperture 308 therethrough, a reaction area 304 and a transfer material 302 associated with the reaction area 304, i.e., lying directly above or on top of the reaction area 304. As can be seen, the fluid transfer material 302 is a unidimensional piece of material. That is, the shape and the dimensions such as the thickness and width of the transfer material 302 are constant throughout the entire structure. Typically, the transfer material is generally fabricated to have a thickness of about 0.020 to 0.030 inches, a width of about 0.20 to 0.30 inches and a length of about 0.90 to 1.10 inches.
Typically, a patient obtains physiological sample such as blood, blood fractions or interstitial fluids, from a finger or arm puncture site, where the volume of sample obtained from such a puncture may vary considerably depending on the particular patient, the sampling site and the like. Sample is applied first to the transfer material or structure in communication with the reaction area of the test strip and then a portion of the sample is then filtered through to the reaction area. The transfer material is usually configured and sized to retain or hold excess sample so that the excess sample does not contaminate other portions of the test strip or contaminate portions of an automatic device into which the test strip is inserted for automatically performing the testing processes. Such contamination may cause false or inaccurate results.
Thus, this transfer material assists in sample collection and helps to dissipate or spread the sample evenly over the reaction area, retain excess sample and may further serve to filter our or exclude unwanted components in the sample before they reach the reaction area. Although this material plays an important role in sample transfer to the reaction area, it has certain disadvantages associated with it. First and foremost, to transfer sample through the material to the reaction area, the portion of the material over the reaction area must first reach saturation, where the volume of sample needed to saturate the material is much greater than what is required by the reaction area to perform an accurate test. Usually, a sample volume of about 7 to 50 microliters and more usually about 7 to 10 microliters is needed to saturate the filter or transfer material of currently configured test strips, however only 1 to 3 microliters is actually needed at the reaction area. Thus, it will be apparent that this transfer material determines the volume of sample that is required from the patient, not the actual volume needed by the reaction area to perform an accurate test.
This rather larger volume of sample needed to saturate this material is often difficult to obtain from a patient. For example, obtaining this volume may require the use of a large diameter needle and/or deeper penetration into the skin. Even if a large diameter needle is used and/or a needle has been penetrated deep into the skin, oftentimes, a first puncture produces insufficient volume for the particular test being performed and thus the skin must be punctured again until a sufficient volume is ultimately obtained. These factors can increase discomfort and pain felt by the patient, and may be extremely difficult to achieve for those individuals whose capillary blood does not readily express. As this sampling process may require repeating frequently within a single day, for many patients, the pain associated with sample collection quickly becomes less tolerable or intolerable all together.
Furthermore, conventional test strip configurations using a material to transfer sample to the test strip require the sample be applied directly to the center of the transfer material or top of the test strip. In other words, the patient must either (1) hold the test strip with the transfer material facing up and turn a finger toward the material so that the sample drop expressed therefrom goes downward onto the strip or, alternatively, (2) position the strip, transfer material side down, onto a finger with a sample drop facing upward. Either way, the patient's view of the material is obscured, blocking the view of how much sample has been applied to the material and thus how much more is needed until the material is saturated. This disadvantage often results in patients applying a volume of sample greater than that which is required, further contributing to the pain and discomfort associated with sample collection.
As such, there is continued interest in the development of new devices and methods for use in the determination of analyte concentrations in a physiological sample. Of particular interest would be the development of such devices, and methods of use thereof, that require minimal sampling volumes, i.e., the transfer material possesses small void volumes, enable the dissipation or spread of the sample evenly over the reaction area, retain excess sample, filter unwanted components in the sample before they reach the reaction area, are easy to use and easy to manufacture.
Relevant Literature
References of interest include: U.S. Pat. Nos. 5,515,170 and 6,168,957 B1.